The synchronous speed is the speed of the rotating magnetic field formed inside the electric motor and through it one can know the value of the rotation of the motor.
A little lower than the synchronous speed, we have the asynchronous speed, which is the rotation measured in the motor shaft. In short, it is the true rotation of the engine, discounting the losses. Hence the asynchronous motor name, since asynchronous means out of sync, in the case between the speed of the magnetic field is the speed of the motor shaft. The value read on the motor nameplate, the nominal value is the value of the asynchronous speed.
Slippage is the difference between the velocity of the magnetic field = synchronous speed and the rotation of the motor, 41bs36 5/8 martin also called slippage. The slipping of an engine normally varies depending on the load. Explaining better: when the load is zero (empty engine) the slip will be practically zero; when it is nominal, the slip will also be the nominal.
Slipping can be given in rpm or%. Another consideration that should be taken into account is that slippage decreases as the rated power of the motor increases.